1. Field of the Invention
The present invention relates to a liquid crystal display device.
2. Background Art
In a liquid crystal display device, a liquid crystal layer is sandwiched between a transparent substrate placed on the side of an observer and a transparent substrate that faces the other transparent substrate and is placed on the opposite side from the observer. The polarization state of light being transmitted through the liquid crystal layer is controlled in accordance with the electric field induced between electrodes provided on the inner sides of the respective substrates. A pair of polarizing plates is also provided: one is placed on the observer's side of the two substrates, and the other is placed on the opposite side of the two substrates from the observer's side.
Liquid crystal display devices are classified into several modes, depending on the initial alignment state of the liquid crystal layer and on the operating state and alignment state of the liquid crystal layer at the time of application of voltage. For example, the VA (Vertical Alignment) mode is used in liquid crystal display devices used for liquid crystal television sets and in liquid crystal display devices for vehicles such as instrument panels (see Japanese Patent Application Laid-Open Nos. 5-113561 and 10-123576, for example).
In the VA mode, the contrast ratio is high when the display is viewed from the front, and the viewing angle is wide. Accordingly, the VA mode allows excellent visibility.
In the VA mode, the liquid crystal layer sandwiched between the substrates is a nematic liquid crystal layer having negative dielectric constant anisotropy (Δ∈) substantially perpendicular to the substrates in the initial alignment state (vertical alignment). A pair of polarizing plates is placed to sandwich the liquid crystal layer, so as to form a crossed Nicol prism in the normal state. When a voltage is applied to the liquid crystal layer via electrodes, the alignment of the liquid crystals is changed and is tilted in accordance with the field intensity, so that the liquid crystal molecules become perpendicular to the electric field or the alignment direction of the liquid crystals become parallel to the substrates. As a result, a difference in the light transmission properties determined by the product (Δn·d) of the refractive index anisotropy (Δn) of the liquid crystals and the thickness (d) of the liquid crystal layer, particularly a difference in color, appears between the portion to which the voltage is applied and the portion to which the voltage is not applied. In a liquid crystal display device, desired display is performed by taking advantage of such characteristics.
As described above, in the VA mode, the liquid crystal layer has negative dielectric constant anisotropy. Therefore, liquid crystals having negative dielectric constant anisotropy are used. Known examples of such liquid crystals include those containing a cyano group in the molecular structure thereof. However, such liquid crystals have a high viscosity, and the high viscosity significantly lowers the response speed of the liquid crystals while the liquid crystal display device is being operated. Such liquid crystals have low reliability and poor heat-resisting properties. Further, it is difficult to remove impurities from such liquid crystals. As a result, the performance and stability of the liquid crystals are easily degraded. Meanwhile, liquid crystals containing fluorine atoms in the molecule structure thereof are also known to have negative dielectric constant anisotropy. Such liquid crystals have a low viscosity, and the low viscosity can increase the response speed of the liquid crystals while the liquid crystal display device is being operated. Such liquid crystals have high reliability and excel in heat-resisting properties. Further, it is easy to purify such liquid crystals. Accordingly, in the VA mode, fluorine-containing liquid crystals are selected, and a liquid crystal layer is normally formed by using the fluorine-containing liquid crystals.
When the protection resin film formed on the surface of a liquid crystal display device is peeled off, or when an electrically charged user touches a liquid crystal display device, static electricity is generated in the surface of the liquid crystal display device. A voltage generated by the static electricity is then applied to the liquid crystal layer, and the alignment of the liquid crystals is changed. As a result, a portion that is not originally intended to be displayed is displayed (this phenomenon will be hereinafter referred to as a “display defect”). Particularly, a liquid crystal layer formed by using fluorine-containing liquid crystals has a high resistivity, since the liquid crystals have a high purity. Therefore, it is difficult to release the static electricity through the liquid crystal layer. As a result, a display defect often occurs in the liquid crystal display device.
Display defects are affected by the electrode patterns of liquid crystal display devices.
For example, in a liquid crystal television set of the active matrix type, a dot matrix structure is used. This structure has a relatively high aperture ratio, and electrodes are densely arranged in the display unit. Therefore, even if static electricity is generated in the surface of such a liquid crystal display device, the static electricity can be scattered through the electrodes. Also, an abnormally lighted portion or a portion that is not originally intended to be displayed is covered with the black mask of color filters. Accordingly, display defects can be made difficult to be seen from an observer.
On the other hand, in a liquid crystal display device of the passive matrix type that can display characters, the problem of display defects due to static electricity can easily become evident. A structure used for displaying characters is not a dot matrix structure in which electrodes are densely arranged. In the structure used for displaying characters, the aperture ratio of the display unit is 70% or lower, and the area in which electrodes are not formed is large in the display unit. Therefore, it is difficult to scatter static electricity through electrodes, and direct-current charges are localized to cause a display defect due to abnormal lighting. From the viewpoint of an observer, portions that are originally intended to be displayed and portions that are not originally intended to be displayed are both displayed on the screen. This results in poorer display performance.
Furthermore, many liquid crystal display devices of the passive matrix type do not use color filters. Therefore, to conceal abnormal lighting of a portion that is not originally intended to be displayed, a black mask needs to be additionally prepared, and, as a result, the production costs become much higher.
As one of the techniques for reducing such display defects, adding a substance that reduces the resistivity of liquid crystals to the liquid crystals has been suggested. However, it has already become apparent that sufficient restraint of display defects cannot be expected from such a technique.
Also, there has been a suggested structure in which an external ITO (Indium Tin Oxide) electrode is formed on an outer side of one of the substrates in a liquid crystal display device, so that the voltage generated by static electricity is not applied to the liquid crystal layer. For example, a metal frame is provided on the outer rim of the liquid crystal display device, and the external ITO electrode and the metal frame are electrically connected to each other via a conductive elastic material. Since the metal frame is grounded, static electricity can be discharged from the external ITO electrode through the conductive elastic material and the metal frame, even if the static electricity is generated on the side of the one of the substrates.
In the case of a structure having an external electrode, however, a potential difference appears between the external electrode and an electrode provided on the inner sides of the substrates forming the liquid crystal display device, if there is a difference between the externally-connected electrode potential and the mean potential of the drive waveform of the liquid crystals. After the liquid crystal display device is used over a long period of time, the ionic substances in the liquid crystals move to cause internal polarization, and display defects occur even without a voltage generated by static electricity. Such display defects are even more conspicuous at high temperatures.
In some cases, the electric connection of the metal frame serving as the outer rim of the liquid crystal display device to the external ITO electrode is considered undesirable.
The present invention has been made in view of the above circumstances. Specifically, the present invention provides a liquid crystal display device that can restrain the occurrence of display defects even in a structure having a difference between an externally connected electrode potential and the mean potential of the drive waveform of liquid crystals.
Other challenges and advantages of the present invention are apparent from the following description.